[Merge] ~binli/ubuntu/+source/gdm3:ubuntu/bionic into ~ubuntu-desktop/ubuntu/+source/gdm3:ubuntu/bionic

[Bin Li]

@Iain,

 Done, I thought I forgot commit the DEP-3 changes, and I re-added them, please help check it again, thanks!
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https://code.launchpad.net/~binli/ubuntu/+source/gdm3/+git/gdm3/+merge/354933
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